目录:
ThreadLocal是什么
ThreadLocal是一个能创建线程局部变量的类。通过ThreadLocal提供的get和set方法,可以为每一个使用该变量的线程保存一份数据副本,且线程之间是不能相互访问的,从而达到变量在线程间隔离、封闭的效果。
使用例子
public static void main(String[] args) throws InterruptedException {
final ThreadLocal threadLocal = new ThreadLocal<>();
threadLocal.set("AAA");
new Thread(new Runnable() {
@Override
public void run() {
threadLocal.set("BBB");
System.out.println("get in " + Thread.currentThread().getName() + " " + threadLocal.get());
}
}).start();
Thread.sleep(1000);
System.out.println("get in main thread " + threadLocal.get());
}
执行结果:
get in Thread-0 BBB
get in main thread AAA
首先,在主线程中初始化了ThreadLocal,并且操作的变量是String类型,在主线程中设置该变量为"AAA",主线程等待1秒钟,同时启动了一个子线程也调用ThreadLocal设置该变量为"BBB"并输出,1秒之后通过get输出主线程的结果,发现子线程设置的值并没有影响主线程中设置的值,即通过ThreadLocal修饰的变量可以实现在各个线程之间互不干扰,相互隔离的效果。
源码解析
初始化
//1
final ThreadLocal threadLocal = new ThreadLocal<>();
threadLocal.set("AAA");
//2
final ThreadLocal threadLocal = new ThreadLocal() {
@Override
protected String initialValue() {
return "AAA";
}
};
对应的源码:
protected T initialValue() {
return null;
}
public ThreadLocal() {
}
ThreadLocal的初始化可以有上面1、2两种方式,一种是先初始化然后通过set设置值,一种直接重写initialValue并设置值。既然ThreadLocal可以做到变量的线程封闭,我们有理由猜想是不是ThreadLocal
set值
public void set(T value) {
//获取当前线程
Thread t = Thread.currentThread();
//根据当前线程获取ThreadLocalMap,注:ThreadLocalMap内部并不是通过map来存储value,而是通过数组存储的
ThreadLocalMap map = getMap(t);
if (map != null)
//不为空,内部直接通过数组设置Entry元素(Entry中包装了ThreadLocal及value,其中key=ThreadLocal,value=传入值value)
map.set(this, value);
else
//为空,则初始化一个ThreadLocalMap,并将ThreadLocal及value包装成Entry放入数组中。
createMap(t, value);
}
ThreadLocalMap getMap(Thread t) {
//threadLocals是Thread类中的成员变量
return t.threadLocals;
}
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
Thread类:
public class Thread implements Runnable {
/* ThreadLocal values pertaining to this thread. This map is maintained by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
}
所以set方法首先根据当前线程获取线程中的threadLocals变量(ThreadLocalMap类型),并将ThreadLocal及value包装成Entry放入数组中,因为threadLocals是Thread中的局部变量(存放在栈空间中),所以只有当前线程能访问,其他线程无法访问。这里有个问题:为什么还需要将ThreadLocal作为key传入到ThreadLocalMap呢?因为一个线程中可以初始化多个ThreadLocal,是一对多的关系,所以需要传入ThreadLocal,如果初始化了多个ThreadLocal,根据不同的ThreadLocal可以获得对应的value。那么ThreadLocalMap内部到底是怎么存储的呢?
ThreadLocal静态内部类ThreadLocalMap:
static class ThreadLocalMap {
//内部类Entry,继承了弱引用WeakReference,使用ThreadLocal作为键值
static class Entry extends WeakReference> {
Object value;
Entry(ThreadLocal> k, Object v) {
super(k);
value = v;
}
}
//初始容量 必须是2个倍数
private static final int INITIAL_CAPACITY = 16;
//Entry数组,必要时可以扩容,
private Entry[] table;
//数组大小
private int size = 0;
//初始化ThreadLocalMap,并将ThreadLocal、firstValue封装成Entry并放入Entry数组中
ThreadLocalMap(ThreadLocal> firstKey, Object firstValue) {
table = new Entry[INITIAL_CAPACITY];
int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
table[i] = new Entry(firstKey, firstValue);
size = 1;
setThreshold(INITIAL_CAPACITY);
}
//根据key(ThreadLocal类型)的hash获取Entry在数组中的位置,有数据的话直接返回该数据
private Entry getEntry(ThreadLocal> key) {
int i = key.threadLocalHashCode & (table.length - 1);
Entry e = table[I];
if (e != null && e.get() == key)
return e;
else
return getEntryAfterMiss(key, i, e);
}
private Entry getEntryAfterMiss(ThreadLocal> key, int i, Entry e) {
Entry[] tab = table;
int len = tab.length;
while (e != null) {
ThreadLocal> k = e.get();
if (k == key)
return e;
if (k == null)
expungeStaleEntry(i);
else
i = nextIndex(i, len);
e = tab[I];
}
return null;
}
//根据key(ThreadLocal类型)设置value值
private void set(ThreadLocal> key, Object value) {
Entry[] tab = table;
int len = tab.length;
//获取数组中存取位置
int i = key.threadLocalHashCode & (len-1);
for (Entry e = tab[I];
e != null;
e = tab[i = nextIndex(i, len)]) {
ThreadLocal> k = e.get();
//如果key值在Entry中存在,那么直接覆盖之前的值
if (k == key) {
e.value = value;
return;
}
if (k == null) {
replaceStaleEntry(key, value, i);
return;
}
}
tab[i] = new Entry(key, value);
int sz = ++size;
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
}
//移除key对应的value
private void remove(ThreadLocal> key) {
Entry[] tab = table;
int len = tab.length;
int i = key.threadLocalHashCode & (len-1);
for (Entry e = tab[I];
e != null;
e = tab[i = nextIndex(i, len)]) {
if (e.get() == key) {
e.clear();
expungeStaleEntry(i);
return;
}
}
}
}
get值
public T get() {
//获取当前thread
Thread t = Thread.currentThread();
//根据当前线程获取ThreadLocalMap,注:ThreadLocalMap内部并不是通过map来存储value,而是通过数组存储的
ThreadLocalMap map = getMap(t);
if (map != null) {
//根据this(ThreeadLocal)获取数组中对应的Entry,不为空直接取出value
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
T result = (T)e.value;
return result;
}
}
//如果线程中的ThreadLocalMap为空,则进行初始化
return setInitialValue();
}
private T setInitialValue() {
//初始化值 默认是null
T value = initialValue();
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
return value;
}
ThreadLocal在Handler中的使用
Handler机制:
Handler构造函数:
public Handler(Callback callback, boolean async) {
......其他代码......
mLooper = Looper.myLooper();
if (mLooper == null) {
throw new RuntimeException(
"Can't create handler inside thread that has not called Looper.prepare()");
}
mQueue = mLooper.mQueue;
mCallback = callback;
mAsynchronous = async;
}
Looper.prepare初始化:
static final ThreadLocal sThreadLocal = new ThreadLocal();
public static void prepare() {
prepare(true);
}
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
Android中Handler机制在项目中使用的很频繁,Handler底层通过MessageQueue和Looper来实现消息的线程间通信。其中Handler来发送及接收并处理消息,MessageQueue接收Handler发来的消息,并在Looper循环中根据msg.target(handler)来分发消息。一个线程只对应一个Lopper,一个Looper对应一个MessageQueue,但是一个线程中可以有多个Handler。因为一个线程只能对应一个Looper,且Looper跟线程是一一绑定关系,此时用ThreadLocal再合适不过。
Looper中使用ThreadLocal关联Looper,使得Looper只能在各自线程使用,并且不管handler从哪个线程传来消息,ThreadLocal保证了最终消息在Looper初始化时所在的线程处理。
总结
- ThreadLocal存储变量副本实际是保存在每个线程的threadLocals(ThreadLocal.ThreadLocalMap类型)变量中。
- ThreadLocal包含的对象(指的是ThreadLocal
中的T对象)在不同的线程中有不同的副本(实际上也是不同的实例) - ThreadLocalMap中的Entry弱引用于ThreadLocal,同时也会回收key为null的Entry,从而避免了Entry无法释放导致内存泄漏
画一个简易图:
参考
【1】https://droidyue.com/blog/2016/03/13/learning-threadlocal-in-java/
【2】https://www.cnblogs.com/dolphin0520/p/3920407.html
【3】http://www.jasongj.com/java/threadlocal/
【4】https://juejin.im/post/5ba64dcee51d4543e609656d